1. Field of the Invention
The present invention relates to an image reading apparatus and an image reading method and, in particular, to an image reading method in which an image is read by photoelectrically converting incident light from the image in units of single pixels when the image to be read has been divided into a plurality of pixels and to an image reading apparatus in which the above image reading method can be applied
2. Description of the Related Art
Conventionally, an image scanner is known in which an image is read (i.e. image data representing density values of each pixel in an image) in the following manner. Light emitted from a light source and transmitted through an image recorded on a photographic film or the like is measured (photoelectrically converted) in units of single pixels by a charge accumulation sensor (for example, a CCD). Photometric signals output from the CCD through an electronic circuit constructed so as to include an amplification circuit are then amplified and the amplified photometric signals are converted into digital data by an A/D converter.
In this type of scanner, generally (in the first reading method), the amount of light from the light source is adjusted (the amount of light for each component color is adjusted for a color scan) so that photometric values obtained from incident light when no photographic film has been set in place substantially conform to the maximum photometric value and so that no saturation of the values occurs. The amplification factor of the amplification circuit for amplifying the photometric signals output from the CCD is also adjusted and, image reading is performed after the CCD charge accumulation time has been adjusted (this is sometimes adjusted for each component color in a color scan).
In the first reading method, the dynamic range DR of analog photometric signals output from the amplification circuit is found byDR=Vsat/Vdrkwhen Vsat is the maximum level and Vdrk the black level of the photometric signals. In order to read an image at a wider dynamic range, the black level Vdrk may be reduced and the maximum level Vsat increased, however, the black level Vdrk, in particular, is dependent on: (1) the dark current output from the CCD; (2) noise output from the CCD; (3) the drift of the amplification circuit; and (4) noise output from the amplification circuit. Consequently, the above (1) to (4) are factors that inhibit the widening of the dynamic range when reading a photographic film. (1) and (3) out of the above (1) to (4) can be substantially removed by correcting the dark current (i.e. by correcting the level of the photometric signals by the amount of the difference between the ideal level of the photometric signals when reading optical black (normally 0) and the actual level thereof.
When dark current correction is performed, because the black level Vdrk is replaced by the noise level of the CCD and the amplification circuit Vnoi, the dynamic range of the photometric signals is found byDR=Vsat/VnoiAccordingly, in order to widen the dynamic range of a reading in a scanner with the above structure, it is necessary to reduce (2) the noise output from the CCD and (4) the noise output from the amplification circuit in addition to performing dark current correction. Thus it is necessary to use a CCD having low noise and high performance and to design an amplification circuit also having low noise. The problem is thus that costs are high.
Moreover, when the analog section of a scanner having a CCD and an amplification circuit is designed to have a wide dynamic range, it is also necessary to use an A/D converter which separates and converts the level of input signals into multibit data as the A/D converter for converting photometric signals into digital data. However, the cost of the A/D converter increases the greater the number of multibits. In particular, when dealing with image data comprising a plurality of pixels such as that from an image scanner, high speed analog digital conversion is demanded. As a result, the analog digital converter ends up being extremely expensive. Accordingly, currently, the specifications of each section of an image scanner are determined so that the widest dynamic range possible under the constraints of cost is obtained. Consequently, the performance of the scanner (i.e. the photometric dynamic range and the image reading speed possible from the analig—digital conversion speed) is not always satisfactory.
Further, high performance negative scanners are also known which read negative images with a high level of accuracy by separating negative images recorded on a color negative film into a plurality of pixels (for example, 1000 pixels) and separating each pixel into each component color and measuring the light thereof in order to determine exposure conditions used when a photograph printer exposes the images onto a photosensitive material such as photographic paper or the like. In this type of high performance negative scanner, the light of each negative image is preliminarily measured under photometric conditions in which it is certain that saturation will not occur (prescan) and the density of the lowest density pixel in the negative image is detected. A main photomeasurement (fine scan) is then performed in which the charge accumulation time of the CCD is adjusted for each of the negative images (adjusted for each component color in a color scan) so as to be the longest possible time without the output being saturated by the light from the lowest density pixels, thus ensuring the maximum dynamic range (second reading method).
In the second reading method, often the density of the lowest density pixel is comparatively high relative to, for example, an over exposed negative image which has high density. Therefore, the charge accumulation time for a fine scan is adjusted so as to be long. Moreover, often the density of the lowest density pixels is comparatively low (namely, is close to or identical to the film base density) relative to an under exposed negative image which has a low density. Therefore, the charge accumulation time for a fine scan is also adjusted so as to be short.
Because the gradient of the change in the density relative to the change in the exposure amount in a negative film is small (γ<<1), the gradation of a negative image is a soft gradation and the contrast of the negative image is low. Moreover, because the above high performance negative scanner uses a CCD having a comparatively rough photometric point density (pixel density), the contrast of the light incident on the CCD from each pixel of the negative image becomes still lower. As a result, by adjusting the charge accumulation time in accordance with the density of the low density pixels, as in the second reading method, negative images of any state of exposure type (over exposed negative images/normally exposed negative images/under exposed negative images) can each be read at a wide dynamic range.
However, in the second reading method, reading negative images having high contrast at a wide dynamic range such as negative images of scenes photographed using reverse light, negative images photographed using strobe lighting, and negative images in which light sources are contained in the image is difficult. Moreover, the dynamic range of the reading is also insufficient when reading images recorded on reversal film which has a large gradient of the change in the density relative to the change in the exposure amount (γ≈1), or when making high accuracy readings of images which have been separated into a plurality of pixels (for example, several hundreds of thousands of pixels). This is because the contrast of the light incident on the CCD from each pixel of the image is extremely high.